Constant flow control valves have numerous applications in piping networks. For example, in a building air conditioning and heating system, water or other liquid at an appropriate control temperature may be pumped from a central station through a piping network to various heat exchanger units located throughout the building. Some of these heat exchanger units are located relatively close to the central station while others are located much farther away. The fluid pressure applied across inputs and outputs of the respective heat exchanger units varies widely because of factors such as frictional losses inherent in the flow of the liquid through the piping network and the distances the liquid must travel from the central station.
The flow rate in each branch of the network is a direct function of the pressure drop existing across that branch. Two contributors to the existence and magnitude of the pressure drop are line friction and equipment pressure drop. The actual pressure drop in one branch is often different from the original desired, or designed, value, leading to a flow rate in that branch which is different from the desired flow rate. Changes from the desired flow rate in one branch will influence the flow rate in other branches. To obtain the desired flow rate in the various branches of such a network, the network should be hydraulically balanced.
Hydraulic balancing often involves adding additional pressure to one or more branches within the system, a measure which may create wasteful pressure drops therein. Pumps are frequently oversized to provide the additional pressure required to balance the network. Because these pumps are frequently operated at flow rates and pressures other than their optimum performance conditions, wasteful energy consumption results.
Constant flow control valves help alleviate the need for hydraulic balancing. Examples of such control valves are disclosed in U.S. Pat. No. 4,766,928 issued to Golestaneh and U.S. Pat. No. 5,174,330 issued to Golestan et al. Both references disclose a constant flow rate control valve including a movable piston having a plurality of side ports and an orifice on an end wall of the piston. Pressure differential across the valve moves the piston against a resilient spring to expose an appropriate portion of the side port area to maintain a constant flow rate at that pressure differential. There is a minimum pressure differential required for the flow rate to increase for establishing the desired flow rate for the Golestaneh, Golestan et al. and other prior control valves. For some systems, particularly those with long piping runs, even this minimum pressure drop may not be attainable. For high volume flows, reaching this minimum is costing energy usage.
In the previously known constant flow control valves, a variable orifice regulates the fluid flow. In these constant flow control valves, the variable orifice is distributed along the outside of a hollow piston. As the differential pressure increases, a portion of the orifice moves below the shoulder which reduces the flow passage area. For small flow rates, this distribution of the passage along the entire length of the hollow piston requires that it become extremely narrow. Because the port is so narrow, a small variation in width may be relatively significant and cause a significant variation in the flow rate. Also, with this type of valve, there is some fluid that flows between the outside of the piston and the shoulder which is affected by manufacturing variations in the piston and shoulder. For a given manufacturing variation in the diameter of the piston, the variation in the flow rate between the outside of the piston and the shoulder is typically proportional to the perimeter of the piston.
Another problem with prior control valves is that they may cause internal local disturbances near the piston as the fluid passes through the regulating orifice; these disturbances may reduce the energy of the fluid flow. Still another problem with prior control valves is that their parts and orifices may become clogged with fluid borne particles or have deformities which may alter the respective control valve flow characteristics. Furthermore, prior control valves may have parts with critical dimensions which may require the valve to be expensively manufactured by high precision machinery.